The invention relates generally to an apparatus for checking the shape of a part and more particularly relates to an apparatus for checking the shape of a part with a portion that is a cylinder of revolution about an axis, the part bearing on the apparatus at a plurality of points forming a support polygon.
Patent application EP 0 626 559 describes an apparatus with displacable roller supports in accordance with the diameter of the part to be checked.
It is known, for example from U.S. Pat. No. 4,679,330 cited in EP 0 626 559 to employ a sensor or a comparator to inspect the shape concentricity of a part exhibiting a portion formed as a cylinder of revolution. However in order that the measurements effected by such comparator be reliable when the part is rotated around the axis of such cylinder, it is necessary that the support arrangement and the mechanism for driving the part in rotation do not generate undesirable displacements in translation.
It is known to employ one or several abutments which are retractable according to the rotation axis, such abutments being housed in the apparatus. The part is held against the abutment by means such as a slight inclination of the apparatus causing the part to rest against the abutment by simple gravity or again through the use of magnetized abutments if the material of the part is suitable.
The known systems of positioning by abutment all suffer the same defect: they do not permit fine adjustment of the height of the end of the abutment which comes into contact with an end face of the part.
Such end face may exhibit defects of flatness; for example if it terminates the cylindrical portion of the part, such face has perhaps been cut off by a milling cutter turning around an axis which does not intersect with the axis of the part. There results therefrom periodic displacements of the part at each rotation of such part on its support arrangement, this being detrimental to the reliability of the shape measurements of the part, especially in its noncylindrical portion.
According to the invention an apparatus is provided for checking the shape of a part having a portion which is a cylinder of revolution around an axis and a reference end face.
The apparatus includes a base and a support arrangement mounted on the base for receiving the cylinder portion and comprising two V-shaped pieces. A positioning arrangement is provided for positioning the part axially with respect to the reference end face. A mechanism is provided for driving the part in rotation about the axis. The positioning arrangement comprises a rectilinear portion with an hemispheric end providing an abutment for the end face. The rectilinear portion is mounted slidingly on the base along a direction which forms, with respect to the axis, an angle corresponding to the half of the angle of the V-shaped pieces. An adjustment means is provided for adjusting the position of the rectilinear portion in a manner which maintains the abutment point of the hemispheric end with the end face on the axis.
It is known from physics that a solid is in equilibrium only if its center of gravity is located above a support polygon. In the cited documents in which the part rests on cylindrical rollers it is respectively a generatrix of each roller which determines a support rectangle. The plurality of the points of contact by which the part rests on the apparatus forms these two generatrices.
In a specific embodiment of the invention which will be described hereinafter four points of contact are satisfactory.
The positioning arrangement according to the invention permits placing the part in abutment against a point contained in its axis of rotation. In contrast to the prior art this point is adjustable in height which enables adjusting it as a function of the diameter of the cylindrical portion of the part. When the latter is driven by the drive mechanism, the point formed by the abutment is constantly in contact with the same point on the end face of the part. In the prior art the geometric location of the points of contact between the abutment and the end face is constituted by the intersection between such face and a cylinder the radius of which is the distance from the abutment to the axis of the part. In the very frequent cases where the face exhibits a defect in flatness, such intersection is not contained in a plane orthogonal to the axis.
The invention also resolves the problem when the end face is planar but not perpendicular to the axis. In the prior art the location of the contact points between the abutment and the end face is then an ellipse contained in the plane of the end face, centered on the intersection of the axis of the part with the end face.
Whatever be the form of the end face, supposedly not too remote from a plane perpendicular to the axis, the fact of aligning the abutment of the positioning arrangement with the axis of rotation of the part in accordance with the invention suppresses the periodic translation movements of the part.
This enables precise and reliable measurements by known measuring instruments which check the shape of the part, particularly the portions outside the support arrangement such as reliefs.
In a specific embodiment of the invention, maintaining the part against the abutment colinear with the axis is effected by the rotational drive mechanism. In the case where the support arrangement is not a motor, such mechanism may consist of a roller or disc exerting pressure and drive simultaneously, coming from above the cylindrical portion of the part and rotating on an axis exhibiting a slight angle with the axis of the part, in a manner such that the driving has as corollary the maintenance in abutment of the end face.
This type of mechanism has the advantage of being disengageable and authorizing very low rotation speeds.
A problem frequently met with when rotation speeds are very low is the presence of lubricants on the part to be checked. Such lubricants which cause skating of the drive roller or disc must be eliminated: in the prior art by a cleansing prior to the checking.
It has been found that the use of silicone for the periphery of the drive disc coming into contact with the part enables dispensing with such cleansing without incurring drawbacks.
The basic molecular structure of such a silicone is formed by a polymerized chain comprising alternately atoms of oxygen and silicon, two radicals R and Rxe2x80x2 being likewise connected to each silicon atom.
In a specific embodiment the silicone is obtained by mixing a resin with a hardener.
Satisfactory results are obtained when the pot life of the material before polymerization is at least 90 minutes.
In a specific embodiment of the invention the height of the position of the abutment is calculated directly from the diameter of the cylindrical portion of the part. In the case where the support arrangement of the part consists of four points of contact located on two vees (V-shaped pieces) arranged on either side of the center of gravity of the part, each vee being of 90xc2x0, symmetric relative to the vertical line passing through its summit, the height of the axis is determined by the diameter of the cylindrical portion of the part. In each of the planes of the two vees the radii joining the axis to the respective points of contact are perpendicular to the branches of the vee. The angle of each vee being itself also of 90xc2x0, the parallelogram in the plane of one of the two vees joining the points of contact to the summit of the vee and to the intersection of the axis with such plane is a square. The height of the abutment measured by reference to the line joining the summits of the two veer, must thus be equal to the product of the radius of the cylinder by the square root of 2, or again, to the quotient of the diameter of the cylindrical portion of the part divided by this same number. The abutment is then exactly in the extension of the axis of part resting on the two vees.
In a specific embodiment of the invention, calculations involving the square root of 2 are dispensed with by giving the abutment an appropriate configuration.
In such configuration the abutment is formed from a rectilinear portion the profile of which is at choice but adapted to sliding in a sleeve having a complementary profile. Such sleeve is inclined at 45xc2x0 in a vertical plane containing the line of the summits of the vee and the axis.
The rectilinear portion of the abutment is terminated by a hemispheric end, arranged in a manner such that a single point of the hemisphere contacts the end face of the part. This point is also on the axis of the cylindrical portion of the part.
The abutment and its sleeve are fixed to calibration means enabling the positioning of the abutment by simple manipulations, as will be described in the detailed description.
In a second embodiment of the invention, the rectilinear portion of the abutment is fixed to a micrometer screw inclined at 45xc2x0. The advantage of this embodiment is that it suffices to rotate the knurled head of such micrometer screw until the diameter of the cylindrical portion of the part is displayed. One is then liberated from the necessity of manipulating the part in order to position the abutment.